In the graphic arts, it is desirable to produce a four or more color proof to assist a color etcher in correcting a set of color prints prior to using them to produce color plates and also to reproduce the color quality that will be obtained during the printing process. The proof must be a consistent duplication of the half tone, and should neither gain nor lose color. Visual examination of a color proof should show the following characteristics:
1. Defects on the negative.
2. The best color rendition to be expected from press printing of the material.
3. The correct gradation of all colors and whether grays are neutral.
4. The need, if any, for subduing one of the colors and/or giving directions for altering the film negatives before making the printing plates.
Color proofing sheets, for multi-colored printing, have heretofore been made by using a printing press or a proof press while taking all the steps necessary for actual multicolor printing, but such a conventional method of color proofing has been costly and time consuming.
Photographic processes have also been used, especially photographic processes using photopolymers. There are two known types of photographic color proofing methods; namely, the surprint type and the overlay type.
In the overlay type of color proofing method, an independent transparent plastic support is used for producing a print of each color separation film by applying a photosensitive solution of the corresponding color. A plurality of such supports carrying prints of corresponding colors are then superposed upon each other on a white sheet to produce a color proofing sheet. The overlay type of color proofing method has the disadvantage that the superposed plastic supports tend to darken the color proofing sheet, and, as a result, the impression of the color proofing sheet thus prepared becomes vastly different from copies actually obtained by a conventional printing press or a proof press. Its primary advantage is that it is quick and can serve as a progressive proof by combining any two or three colors in register.
In the surprint type of color proofing method, a color proofing sheet is prepared by successively producing prints of different colors from different color separation films, respectively, by utilizing a single opaque support by applying toners, or by applying photosensitive solutions or coatings of photopolymers of corresponding colors on the opaque support in succession. Examples of some of these approaches are U.S. Pat. No. 3,671,236 and U.S. Pat. No. 3,136,637. An advantage of the surprint type of color proof is that the color saturation is not influenced by the plastic support. This method more closely resembles the actual printing process and eliminates the color distortion inherent in the overlay system.
Various processes for producing copies of an image embodying photopolymerization and thermal transfer techniques are known. See U.S. Pat. Nos. 3,060,023; 3,060,024; 3,060,025; 3,481,736; and 3,607,264. In these processes, a photopolymerizable layer coated on a suitable support is imagewise exposed to a process transparency. The surface of the exposed layer is then pressed into contact with the image receptive surface of a separate element and at least one of the said elements is heated to a temperature above the transfer temperature of the unexposed portions of the layer. The two elements are then separated, whereby the thermally transferrable, unexposed, image areas of the composite transfer to the image receptive element. If the element is not precolored, the tacky unexposed image may now be selectively colored with a desired toner. The colored matter preferentially adheres to the clear unpolymerized material. The lamination, exposure, and development steps are carried out for the respective colors in sequence thus making this process time consuming.
U.S. Pat. No. 3,574,049 provides a transfer process for printing a design on a final support which comprises (a) printing a design onto a temporary support, (b) super-imposing the temporary support and the final support, (c) applying heat and/or pressure to the super-imposed structure formed in (b), and (d) separating the temporary support from the final support which retains the printed design, the affinity of the matter of the design for the temporary support being lower than its affinity for the final support.
In U.S. Pat. No. 3,721,557 a method of transferring colored images is claimed which provides a stripping layer coated between the photosensitive element and the support. When the photosensitive layer is exposed to actinic light and developed, the more soluble portions are selectively removed to produce a visible image. The image-carrying support is pressed against a suitable adhesive coated receptor member and subsequently, the carrier support sheet is stripped to accomplish the transfer of the image. A fresh layer of adhesive is applied to the receptor for each subsequent transfer.